1. Field of the Invention
The present invention relates to a sheet feeding device which can be used in an automatic original document transfer device which feeds and transfers a sheet-like original document to be read in an image reading device to an image reading part thereof and like devices, and an automatic sheet transfer device which can be used as an automatic original document transfer device and like device, and further to an image forming device.
2. Description of Related Art
There are various types of sheet feeding devices employed in an automatic original document transfer device which feeds and transfers a sheet-like original document to be read in an image reading device to an image reading part thereof and like devices, among which there is a device of the following type:
The sheet feeding device comprises a supply roller for feeding sheets loaded on a tray for loading sheets, a drive shaft for supporting the supply roller and rotationally driving the same, a roller supporting member which is swingably supported on the drive shaft, a feed roller which is rotatably supported on the roller supporting member, a feed roller transmission mechanism for rotating the feed roller in conjunction with the rotation of the supply roller, and a roller supporting member transmission mechanism provided on the drive shaft for swinging the roller supporting member by the drive shaft, the feed roller being so constructed that during feeding of the sheet, the roller supporting member is swung by the drive shaft via the roller supporting member transmission mechanism, whereby the feed roller is capable of moving from a retracted position to be in contact with the sheets loaded on the tray to feed the top sheet to the supply roller and during standby, the roller supporting member is swung back by the drive shaft via the roller supporting member transmission mechanism, whereby the feed roller is capable of being retracted to the retracted position.
In this type of sheet feeding device, the drive shaft is rotated by the drive force transmitted from the motor via the electromagnetic clutch, and the drive shaft is often disconnected from the motor when the electromagnetic clutch is turned off. The electromagnetic clutch is often turned off when sheets are not being fed. When the electromagnetic clutch is turned off like this, if no swing limitation mechanism for the roller supporting member is provided, the roller supporting member swings by its own weight under gravity and is lowered so that the feed roller comes into contact with the sheets on the tray. This would adversely affect handling of jam and the like.
As for this point, providing a two-way torque limiter on the paper feed roller shaft (drive shaft) is suggested as a mechanism for retaining the feed roller in the retracted position when drive from the motor to the supply roller and feed roller is removed by the electromagnetic clutch.
Moreover, JP2002-249246A describes a technique of allowing the feed roller to remain in the standby position even if the drive from the motor is removed.
Japanese Patent Publication No. 4051832 describes that a mechanism in which a feed roller is lifted when a cover of an automatic original document transfer device is closed.
However, in the method of employing the two-way torque limiter, the feed roller is retained by the two-way torque limiter, and therefore the torque of the torque limiter is applied to the motor as drive load when the feed roller moves from a sheet feeding position to a retracted position, when moving in the opposite direction and during the sheet feeding operation. This is disadvantageous since the size of the motor needs to be increased and power consumption is increased.
Moreover, in the technique described in JP2002-249246A, a torque limiter is used to retain the feed roller in the standby position, and therefore an extra load due to the torque limiter is applied on the motor during feeding of a sheet, which may cause a step-out of the motor and an increase in power consumption.
In the technique described in Japanese Patent Publication No. 4051832, only the cover is opened and closed when handling jam, and the feed roller is in a state of being lowered on the sheet. Handling jam is therefore uneasy.